S1900
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
agreement between measured and calculated dose profile penumbrae for small 6 MV fields of a TrueBeam accelerator (Varian). Furthermore, the impact of ETSS variations on the output factors (OF) was evaluated. Material/Methods: Both crossplane and inplane dose profiles for small fields (1x1, 2x2 and 3x3 cm2) defined by the multileaf collimator (MLC) were measured in a water phantom using a microDiamond detector (PTW), at a source-to- surface distance of 95 cm and a depth of 5 cm. The measured profiles were compared to those calculated by the TPS(Varian Eclipse, v16.1.2), in the same conditions. ETSS was iteratively modified from the default values: (0,0) mm for the AAA algorithm and (1,1) mm for Acuros® XB(AXB). The tested configurations were (0,0) and (1,0) mm for AAA, and (1,1), (1.5,1), (1,0) and (1.5,0) for AXB. The final configurations for each algorithm correspond to vendor recommendations for MLC shaped fields, applicable from Eclipse v13 onwards. The comparison was performed by analysing the profiles' penumbrae using Python-based tools. The analysis comprised: the distance between the 80% relative dose positions of the measured and calculated profiles; the analogous distance for 20% for crossplane and 45% for inplane (the latter selected to minimize the MLC transmission ripple effect); a point-wise mean distance and the slope of the linear approximation of the penumbrae (Graph 1).
(fig 2).
Conclusion: UHDR-RT could achieve substantial NT sparing in breast cancer, reducing doses to all irradiated OARs while maintaining tumor control. The ultra-short beam delivery times may enhance DIBH feasibility, especially for patients with limited breath-hold capacity, supporting broader clinical application. References: 1. Hörberger F, Petersson K, Ceberg S, Bäck S, Adrian G, Ceberg C. Investigating the therapeutic potential of FLASH radiotherapy - a treatment planning study. Radiother Oncol. 2025 Aug. 209:110947. doi: 10.1016/j.radonc.2025.110947. Keywords: Breast cancer, FLASH-RT, Normal tissue toxicity Digital Poster 2764 Enhancing the Beam Model: Fine-tuning the Effective Target Spot Size José Mesquita 1,2 , Íris Almeida 1,2 , Diana L. Silva 1,2 , Anabela G. Dias 1,2 , Joana Lencart 1,2 , Catarina Souto 1,2 1 Medical Physics, IPO-Porto, Porto, Portugal. 2 Medical Physics, Radiobiology and Radiation Protection Group, IPO-Porto, Porto, Portugal Purpose/Objective: This work aimed to improve the accuracy of the Treatment Planning System (TPS) beam model for two dose calculation algorithms by iteratively adjusting the Effective Target Spot Size (ETSS) and evaluating the
Additionally, the effect of applying different ETSS values on OF of 2x2, 1x2, 1x3, 2x1 and 3x1 cm2 fields was assessed.
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